Large Hadron Collider: World's biggest physics experiment restarts

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Photos:Exploring the universe at CERN

The Large Hadron Collider is located at CERN, the European Organization for Nuclear Research, near Geneva, Switzerland. This is CERN's Globe of Science and Innovation, which hosts a small museum about particle physics inside. The ATLAS experiment is housed underground nearby.

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Photos:Exploring the universe at CERN

The Higgs boson, the elusive particle that scientists had hoped to find for decades, was detected by two general-purpose experiments at the Large Hadron Collider, as scientists announced in 2012. The Compact Muon Solenoid (CMS) experiment, pictured, is one of them.

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Photos:Exploring the universe at CERN

The ATLAS experiment, seen here in 2011, also detected the Higgs boson, a particle that helps explain why matter has mass. It has been called the "God particle" because of a book by that title, but scientists hate the name.

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Photos:Exploring the universe at CERN

Much of three stories of electronics at CMS are involved in making split-second decisions about what data to keep and what to discard. This is one of those areas.

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Photos:Exploring the universe at CERN

A technician works on the CMS experiment. Technicians are adding new cooling lines for CMS for a system that will be put in place in two or three years.

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Photos:Exploring the universe at CERN

CMS is adding this layer for the next run of particle collisions to improve the detection of muons, which are fundamental particles.

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Photos:Exploring the universe at CERN

Physicists work in the CMS control room.

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Photos:Exploring the universe at CERN

The nearly 14,000 tons of machinery can all collapse together, or separate, when high-pressure air is pumped in. This is one of the pads to help slide it all around.

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Photos:Exploring the universe at CERN

CMS has 76,000 lead-tungstate crystals that shatter electrons and photons, allowing scientists to observe particles such as the Higgs boson that exist for only an instant. Some of those crystals are in the endcap.

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Photos:Exploring the universe at CERN

Evaldas Juska, an engineer, is working on computers involved with CMS.

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Photos:Exploring the universe at CERN

CMS was constructed at ground level, then pieces of it were lowered through this hole in the cavern.

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Photos:Exploring the universe at CERN

This is the CERN Computing Center. Tim Berners-Lee invented the World Wide Web at CERN.

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Photos:Exploring the universe at CERN

One of the world's first web servers, a NeXT computer from 1991, is seen at CERN. The handwritten note indicates, "This machine is a server. DO NOT POWER DOWN!" On the right is an old Ethernet cable, which can handle only 10 Mb/second, and was largely replaced by the mid-'90s.

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Photos:Exploring the universe at CERN

While they take their work seriously, that doesn't mean the scientists at CERN don't have a sense of humor. Here we see CERN's "Animal Shelter for Computer Mice," where used and unwanted computer mice have a place to call home.

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Photos:Exploring the universe at CERN

A sculpture garden featuring artwork made from pieces of old experiments decorates the grounds at CERN.

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Photos:Exploring the universe at CERN

A collection of empty relics from the celebrations of different milestones of the CMS experiment.

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Photos:Exploring the universe at CERN

"Don't feed the physicists" marks a box of coins where CMS scientists deposit change to pay for coffee.

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Joe Incandela, the spokesperson for CMS, says that about 4,000 scientists work there.

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Story highlights

The Large Hadron Collider (LHC) begins again after a two-year shutdown

The restart was delayed in March

(CNN)The world's biggest and most powerful physics experiment is taking place as you read this.

The Large Hadron Collider (LHC), a particle accelerator and the largest machine in the world, is ready for action following a two-year shutdown.

After problems that delayed the restart in March, scientists at the European Organization for Nuclear Research (CERN) completed final tests, enabling the first beams to start circulating Sunday inside the LHC's 17 mile (27 km) ring.

The LHC generates up to 600 million particles per second, with a beam circulating for 10 hours, traveling more than 6 billion miles (more than 10 billion kilometers) -- the distance from Earth to Neptune and back again. At near light-speed, a proton in the LHC makes 11,245 circuits per second.

Why does it matter?

The purpose of the lengthy project is to recreate the conditions that existed moments after the "Big Bang" -- the scientific theory said to explain the creation of the universe. By replicating the energy density and temperature, scientists hope to uncover how the universe evolved.

The burning questions that remain include the origin of mass and why some particles are very heavy, while others have no mass at all; a unified description of all the fundamental forces such as gravity; and uncovering dark matter and dark energy, since visible matter accounts for only 4 percent of the universe.

The LHC could also question the idea that the universe is only made of matter, despite the theory that antimatter must have been produced in the same amounts at the time of the Big Bang.

CERN says the energies achievable by the LHC have only ever been found in nature.